This invention relates to methods and apparatus for a visual output device which uses electro-optic cells, such as liquid crystal cells. Such output devices are commonly used in calculators, watches, electronic thermometers, or the like.
With the advent of hand-held devices came the need for effective visual output means which could be powered using a lower voltage than the associated desk top devices had access to utilize. The use of an electro-optic cell, e.g., a liquid crystal cell, has been widely accepted in the industry due to its ability to be constantly visible to the user, its low cost, and its relatively low power demand. An electro-optic cell may be characterized as a device which will allow light or a patterned image to pass through the cell, depending on the electrical energy received. Commonly these cells use a liquid crystal material.
For information content higher than the 23 segments in a standard digital watch display, multiplexing the display is essential to avoid excessive chip/display interconnections. Multiplexing involves exciting one pixel and moving quickly to another operation. Given the X-Y matrix used in multiplexing, a given pixel receives exitation signals during the whole addressing cycle, not just at the time it is specifically addressed. In order for the drive technique to work, the pixel in question must not respond to the extra signals. This means that the electro-optic response, transmission function, of the device must have a threshold characteristic. The extent to which the display turns on in a multiplex drive scheme is related to the steepness or slope of the contrast ratio vs. voltage curve above threshold. The sharper this transition, the more lines which can be addressed and consequently the more data that can be presented. For many nonemissive type displays, e.g., liquid crystals, the sharpness of this transfer function is poor, leading to a limited level of multiplexing and a limited information content.